This invention relates to the use of an improved gas detecting element which selectively detects only carbon monoxide from among various reducing gases in air.
It has already been known to use a metallic oxide semi-conductor containing stannic oxide, SnO.sub.2, as the main component for a reducing gas detecting element. The concentration of the gas is detected by utilizing the change in resistance of this element caused by the adsorption of the gas.
However, the above-mentioned conventional element responds to various gases, and furthermore, it must be heated to a high temperature of about 250.degree. C. when in use. Thus, due to the lack of selectivity, and the high consumption of electric current, the conventional element is quite unsuitable for use in a gas alarm which is required to respond only to a specific gas, or which is required to be continuously used.
In order to overcome this drawback, there has been proposed an SnO.sub.2 -Pt type gas detecting element prepared by adding platinum, Pt, to stannic oxide, SnO.sub.2, as a catalyst, which element selectively responds to carbon monoxide gas and works at a relatively low temperature (see Japanese Patent Laid Open No. 50-17287). However, even with regard to this element, the resistance of the element varies in accordance with the change in temperature of the element, and therefore it is necessary to provide a subsidiary circuit in order to maintain a constant temperature of the element. Besides, it is necessary to use the element at a temperature of about 70.degree. C. or higher in order to amplify the resistance change caused by the adsorption of gas.